1. Field of the Invention
The present invention relates generally to a control system for driving an automotive windshield wiper with a desired timing. More specifically, the invention relates to a control for an automotive wiper which drives the automotive wiper with a desired intermittent timing. Most particularly, the invention relates to a control system for an automotive wiper system including a front windshield wiper and a rear windshield wiper.
2. Description of the Background Art
In recent years, a variety of automotive wiper systems have been developed. Some of there wiper systems include not only wipers for a front windshield but also wipers for a rear windshield. In the following disclosure, the wiper for the front windshield will be referred to as the "front wiper", the wiper for the rear windshield as the "rear wiper and the wiper system including both of the front and rear wipers as the "wiper system".
Usually, the control system for such wiper systems has independently operable switches for the front and rear wipers. The switch for the front wiper is generally switchable between a LOW speed mode position, a HIGH speed mode position, an INTERMITTENT mode position, a WASHER mode position and an OFF mode position. The switch for the rear wiper is switchable between an ON position and an OFF position. In such a wiper control system, the rear wiper is driven at low speed continuous mode while the associated switch is held in the ON position. Usually, the amount of the rain water on the rear windshield is much less than that on the front windshield. Especially while the vehicle is running, the amount of rain water on the rear windshield becomes less with increasing vehicle speed. Thus, the driver will usually switch the switch ON and OFF while observing the condition of the rear windshield. The attention required to operate the rear wiper without causing damage to the wiper blade or rear window pane clearly complicates driving.
To free the driver from the rear wiper control operation, it would be preferable to allow control by a single switch operation for selecting the wiper operation mode for the front wiper. Such a wiper control system which allows control of both of the front and rear wiper operation with a single switch has been proposed in Japanese Patent First (unexamined) Publication (Tokkai) Showa 56-13239. In the disclosed wiper control system, a common intermittent control circuit is used for controlling intermittent timing for the front and rear wipers. Therefore, the front and rear wipers are driven at the same intermittent timing. The intermittent timing is usually selected depending upon the rain condition and in view of rain water amount on the front windshield. While the vehicle is not running, the amount of the rain water amount on the front and rear windshield would be approximately equal and by selecting the intermittent timing in view of the amount of rain water on the front windshield, a thin water layer necessary for smooth operation of the wiper can be established on the rear windshield. However, while the vehicle is running, air flows along the external surface of the vehicle body and prevents rain drops from falling on the rear windshield. As a result, the rain water amount on the rear windshield becomes less than that on the front windshield. Even in this condition, in the prior art system, the front and rear wipers are driven at common intermittent timing. Therefore, the amount of water on the rear windshield becomes insufficient for establishing a thin water layer for smooth wiper operation. Failure to establish the water layer, results in higher friction between the wiper blade and the windshield surface, tends to cause scratches on the windshield surface and, in turn, accelerate wearing of the wiper blade, shortening wiper blade life.